A conventional reflecting telescope, it will be appreciated, employs a system of mirrors contained within an optical tube to gather and focus rays of light at a focal point. Typically, a primary mirror gathers and reflects light to a secondary mirror that reflects and focuses the light at a focal point. The light reflected from the secondary mirror can be viewed through an eyepiece. The telescope is focused by causing the optical tube focal point to coincide with the focal point of the eyepiece. A photographic plate or CCD sensor may be used instead of an eyepiece in certain applications to "view," or capture the light reflected from the secondary mirror.
Effecting relative movement between the focal points of the optical tube and eyepiece is the general method used to focus a reflecting telescope. This may be accomplished by moving the primary and secondary mirrors to change the position of the optical tube focal point. In this method, the optical tube and eyepiece remain in fixed positions relative to one another. A problem with this method is that moving the optical tube mirrors can cause the viewed image to shift, requiring that the telescope be re-aimed in order to view the original image.
Various mechanisms have been designed that allow a telescope to be focused without moving the optical tube mirrors. These include sliding drawtubes, threaded focusing tubes, rack and pinion systems, all of which are configured to move an eyepiece longitudinally relative to an optical tube. An example of such a focussing apparatus may be found in U.S. Pat. No. 5,303,090, the disclosure of which is incorporated herein by this reference thereto.
Known optical focuser designs are effective under most circumstances, though they do suffer from various problems and disadvantages. Some designs require use of components machined to within precise tolerances, making the focuser costly to manufacture and difficult to maintain. Where equipment does not meet these tolerances, whether due to poor construction or to inevitable wear, the telescope may suffer from problems related to jamming, rocking, or slippage of the moving parts. This, in turn, may result in poor alignment of the eyepiece relative to the optical tube. These failures can lead to difficulty in bringing the telescope into focus.
A problem common to drawtube designs is that they typically require supporting structures surrounding the drawtube to hold the drawtube aligned as it moves with respect to the optical tube. These supporting structures can decrease access to the drawtube, making it more difficult to integrate certain types of optical components into the focuser. For example, a flip mirror system, an optical component frequently used in conjunction with CCD sensors, cannot easily be integrated into known drawtube designs. Instead, the flip mirror system is provided as a separate component that must be affixed to the end of the focuser, increasing the size and weight of the structure attached to the optical tube of the telescope. Bulky and/or heavy attachments to the optical tube may imbalance the telescope, place unnecessary strain on the structure supporting the optical tube, decrease portability, and/or make the telescope difficult to aim or focus.